Nondependent subjects with histories of heroin abuse participated in a study evaluating the influence of rate of administration of a fixed 10 mg dose of morphine sulfate on drug-induced subjective effects of the drug. Morphine or saline was infused intravenously over 120, 30 or 10 seconds to three separate groups. Self-reports of mood and feeling state and static pupillary diameter were monitored for 4 hours after the infusion. Only the 10-second morphine infusion produced significant subjective effects indicative of the positive, rewarding effects of morphine. When asked to identify the infused drug, subjects correctly identified morphine infused over 10 seconds 85 % of the time compared to 38% following the 120-second infusion. Concurrent static measurement of pupillary diameter suggested that the degree of pupil constriction did not change with the infusion rate, indicating a dissociation of subjective and miotic effects. These data indicate that faster rates of administration produce subjective effects associated with greater abuse liability. - morphine, subjective effects, dependence - Human Subjects